1. Field of the Invention
The present invention relates to an electrophotographic photoconductor and a method for producing the same. In particular, the present invention relates to an electrophotographic photoconductor comprising an undercoat layer provided between a conductive substrate and a photosensitive layer and a method for producing the same.
2. Description of the Prior Art
In general, the electrophotographic process using a photoconductive member has been used as one of the information recording means utilizing the photoconductive phenomenon caused by the photoconductive member. In this process, an image is formed in the following manner. That is, the photoconductive member is placed in the dark and charged of its surface by means of corona discharging, and the photoconductive member is exposed with light so that the charges on the exposed area are erased selectively. Thus, a latent electrostatic image corresponding to the unexposed area is formed. Subsequently, colored, charged fine particles (toner) are deposited on the latent electrostatic image by means of electrostatic attractive force or the like. Thus, the latent image is developed to form a visible image. In such electrophotographic techniques, the photoconductor used is required to have the following basic characteristics:
(1) it can be charged uniformly at an appropriate potential in the dark; PA1 (2) it can have a high charge-retentivity or the charges present thereon are hardly discharged in the dark; PA1 (3) it has an excellent photosensitivity and, therefore, can erase the charges thereon rapidly; PA1 (4) the surface of the photoconductor can be discharged by light irradiation readily; PA1 (5) it exhibits less residual potential; PA1 (6) it has good mechanical strength and flexibility; PA1 (7) it is stable in various electrical characteristics, particularly charging property, photosensitivity and residual potential under repeated use; and PA1 (8) it has a resistance against heat, light, temperature, humidity, deterioration with ozone and the like so that it has good stability and durability. PA1 (1) To prevent the injection of unnecessary charges from the substrate into the photosensitive layer, so as to prevent the decrease in the surface potential of the photosensitive layer or prevent the generation of image defects during developing; PA1 (2) To cover any defects present on the surface of the conductive substrate; PA1 (3) To improve a charging property of the photoconductor; PA1 (4) To improve an adhesion property of the photosensitive layer; and PA1 (5) To improve a coating property of the photosensitive layer to the conductive substrate. PA1 an conductive substrate; PA1 an undercoat layer provided on the conductive substrate; and PA1 a photosensitive layer having both charge-generation and charge-transport functions provided on the undercoat layer, PA1 (1) dispersing a polyamide resin and methylhydrogenpolysiloxane-treated titanium dioxide in a mixture solvent formed of an alcohol-type solvent and a halogenated hydrocarbon-type solvent, thereby preparing a coating liquid for an undercoat layer; PA1 (2) applying said coating liquid onto a conductive substrate, thereby providing an undercoat layer on the substrate; and PA1 (3) forming a photosensitive layer on said undercoat layer. PA1 (a) forming a charge-generation layer on the undercoat layer; and PA1 (b) forming a charge-transpiring layer on the charge-generation layer.
On the other hand, in the electrophotographic photoconductor currently used (which comprises a conductive substrate and a photosensitive layer provided on the conductive substrate), an undercoat layer has been provided between the substrate and the photosensitive layer for the purpose of satisfying the following requirements.
The resin conventionally used for the undercoat layer is, for example, a resin material such as polyethylene, polypropylene, polystyrene, acrylic resin, vinyl chloride resin, vinyl acetate resin, polyurethane resin, epoxy resin, polyester resin, melamine resin, silicone resin, polybutyral resin, polyamide resin, etc. or a copolymer resin comprising at least two repeating unit members thereof such as vinyl chloride-vinyl acetate copolymer resin, acrylonitrile-styrene copolymer resin, etc. In addition, also used is casein, gelatin, poly(vinyl alcohol), ethyl cellulose, or the like. Among them, especially preferred is polyamide resin. (See Japanese Patent Application Laying-open Nos. 47344/1973, 25638/1977 and 95351/1983.)
However, in the above-mentioned electrophotographic photoconductors using a polyamide resin as the undercoat layer component, since the volume resistivity of the undercoat layer generally lies within the range from about 10.sup.12 to about 10.sup.15 ohm.multidot.cm, the undercoat layer should have a thickness as small as about 0.3 .mu.m or less. In such case, if the thickness of the undercoat layer is thicker than about 0.3 .mu.m, the residual potential is likely to be accumulated in the photoconductor, resulting in the occurrence of fogging on the copied image. On the contrary, if trying to make the thickness of the undercoat layer thinner, it tends to become difficult to control the thickness of the undercoat layer during coating, which may lead to the difficulty in sufficient covering of defects present on the conductive substrate with the undercoat layer, as well as the difficulty in the satisfactory improvement in charging property of the photoconductor obtained.
In these situations, some types of electrophotographic photoconductors have been proposed. For example, for the purpose of reducing the residual potential on the photoconductor and preventing the generation of defects of the image obtained, Japanese Patent Application Laying-open No. 280864/1987 discloses an electrophotographic photoconductor in which 1 to 10 parts by weight of a mixture of titanium oxide and tin oxide is dispersed in 100 parts by weight of nylon-8 resin to prepare an undercoat layer. Furthermore, for the purpose of improving the dispersibility of the titanium oxide in the resin component, Japanese Patent Application Laying-open No. 181158/1990 discloses an electrophotographic photoconductor in which alumina-coated titanium oxide particles are contained in the undercoat layer.
Thus, it has become possible to achieve the production of an electrophotographic photoconductor having a thick undercoat layer and having reduced residual potential by incorporating titanium oxide into the undercoat layer. However, in such photoconductor, a problem that the stability of the photoconductor under repeated use is affected by environmental conditions, particularly low-temperature/low-humidity conditions is still remaining.
For these reasons, there has been demanded an undercoat layer which can contribute to improving the charging characteristics and the residual potential, so that providing a photoconductor showing less accumulation of residual potential and less decrease in photosensitivity under repeated use and various environmental conditions. Accordingly, it is required for the resin material of the undercoat layer not to cause the aggregation of the charge-generation substance in the photosensitive layer when contacting with the photosensitive layer. It is because the aggregation of the charge-generation substance results in the non-uniform coating of the photosensitive layer onto the undercoat layer, which may cause the decrease in photosensitivity of the photoconductor and the non-uniformity in the contrast of image obtained. In addition, it is further required for the undercoat layer to have stable resistivity over wide environmental conditions including low-temperature/low-humidity and high temperature/high humidity conditions. Furthermore, it also be required for the undercoat layer to form a barrier against the injection of positive holes from the conductive substrate into the photosensitive layer and to have a resistance against solvents used for preparation of the photosensitive layer.
In the preparation of the coating liquid for the undercoat layer, an alcohol-type solvent such as methyl alcohol and ethyl alcohol has conventionally been used, since the main component of the undercoat layer is an alcohol-soluble nylon in most cases. However, the use of the solvent composed of alcohol-type solvent alone has a problem of non-uniformity in coating of the coating liquid onto the conductive substrate due to the non-uniform evaporation of the solvent after the coating. When the photosensitive layer contacts with the non-uniformly coated portion of the undercoat layer, the charge-generation substance in the photosensitive layer comes to cause aggregation at the portion, resulting in non-uniform coating of the photosensitive layer or the undercoat layer. These phenomena cause the decrease in photosensitivity of the photoconductor and the non-uniformity in the contrast of the copied image, which are disadvantageous.